Add basic Google Custom Search to Hugo docs

Fixes #753

docs/layouts/partials/google-cse.html

@@ -0,0 +1,13 @@
+<script>
+  (function() {
+    var cx = '004940163756227230940:9tdbvl9bwk4';
+    var gcse = document.createElement('script');
+    gcse.type = 'text/javascript';
+    gcse.async = true;
+    gcse.src = (document.location.protocol == 'https:' ? 'https:' : 'http:') +
+        '//www.google.com/cse/cse.js?cx=' + cx;
+    var s = document.getElementsByTagName('script')[0];
+    s.parentNode.insertBefore(gcse, s);
+  })();
+</script>
+<gcse:search></gcse:search>

docs/layouts/partials/header.html

@@ -81,3 +81,4 @@
                               <!--<h3>{{.Title}}</h3>-->
                           <!--</header>-->
                     <div class="panel-body">
+{{ partial "google-cse.html" . }}